Low density foam is generally defined as having a density of about 20 lbs./ft.sup.3 and below. Polyvinyl chloride alone cannot be used to make low density foam since it contains 5 to 10% crystallinity that does not melt completely until 215.degree. C. Because of its poor thermal stability, polyvinyl chloride cannot be processed at 215.degree. C. for long periods of time. Even at lower temperatures, where polyvinyl chloride can be more suitably extruded, it makes poor foam because the crystallites act as physical crosslinks to restrict growth of the cells during expansion, thus yielding high density foam. With increasing blowing pressure in the cells, which may be due to a larger volume of a blowing agent, the polyvinyl chloride structure is ruptured resulting in collapse of the foam, which makes it impossible to make low density foam.
Attempts have been made to overcome the problems created by the presence of crystallites in polyvinyl chloride. U.S. Pat. No. 2,901,446 to Hawkins discloses low density polyvinyl chloride foam that is prepared from a composition comprising polyvinyl chloride admixed with a blowing agent and a solvent boiling above 80.degree. C. The purpose of the solvent is to solubilize the crystallites in polyvinyl chloride and render it more nearly amorphous so that low density foam can be made therefrom. It should be noted that the Hawkins patent discloses the use of azodicarbonamide as the blowing agent, among many others listed in column 2. The blowing agents disclosed by Hawkins are primary blowing agents, as distinguished from nucleating agents, that provide all of the gas required for expanding polyvinyl chloride.
The Landler et al U.S. Pat. No. 3,267,051 discloses a foaming composition comprising polyvinyl chloride, a polyisocyanate, a swelling or blowing agent, at least one vinyl monomer, an ethylenic anhydride, and a saturated halogenated hydrocarbon. Azodicarbonamide is disclosed at top of column 3 as a suitable swelling or blowing agent, along with a number of chlorofluoroalkanes, such as trichlorofluoromethane, as examples of suitable saturated halogenated hydrocarbons. Although the Landler et al patent discloses a combination of azodicarbonamide and trichlorofluoromethane in a polyvinyl chloride composition, this composition also includes a polyisocyanate, at least one vinyl monomer, and an ethylenic anhydride. As is described below, the chlorinated polyvinyl chloride compositions disclosed herein, although they do contain a chlorofluoroalkane as a primary blowing agent and azodicarbonamide as a nucleating agent, do not require the presence of either a polyisocyanate, a vinyl monomer, or an ethylenic anhydride to make low density foam.
The Kraemer et al U.S. Pat. No. 3,366,580 discloses foaming of chlorinated polyvinyl chloride using a chlorofluoroalkane as a primary blowing agent and numerous nucleating agents disclosed at about the middle of column 5. It should be noted that azodicarbonamide is not disclosed as a suitable nucleating agent although sodium bicarbonate and citric acids are, which, as will be shown, are ineffective to produce the desired cell structure.